1. Field of the Invention
The present invention relates to the art of die casting of metals. More particularly, the present invention relates to the design of molds for the die casting of metals. Still more particularly, the present invention relates to the design of overflow gates and overflow chambers in molds for the die casting of metals.
2. Description of Related Art
In the die casting of metals, metal that has been heated to become molten is forced into at least one cavity in a die or mold under pressure. The die typically includes at least two mating parts that can be separated to facilitate removal of the cast part when it has cooled sufficiently. For the purpose of simplicity, the die or mold will be described herein with reference to two mating parts or halves. Until the metal is cool enough to be removed, the die halves must be held together under pressure by a clamping force. The clamping force is often extremely high in order to overcome the force exerted by the molten metal as it is driven into the cavity and thereby keep the die halves effectively sealed.
It is known that the quality of the casting improves with more rapid flow of the molten metal into the die cavity. With conventional control systems, however, as the velocity of the flow of the molten metal into the cavity increases, the pressure that the metal exerts on the die increases. At some point, that pressure might be sufficiently high to overcome the clamping force, causing the die halves to separate and allow metal to leak from the cavity, thereby destroying the integrity of the cast part. As a result, the pressure within the die cavity must be controlled in the die casting process.
To illustrate the details surrounding the problem of pressure control in die casting, FIG. 1 shows a schematic arrangement of the major components typically associated with the die casting process. A hydraulic accumulator 10 pressurizes hydraulic fluid that is transferred through an inlet throttle valve 12 to a hydraulic cylinder 14. Adjustment of the inlet throttle valve 12 and an outlet throttle valve 16 allow the movement of an actuating piston 18 to be controlled. The actuating piston 18 is secured to an intermediate connection 20, which allows the actuating piston to drive a plunger 22.
The plunger 22 terminates with a plunger tip 24 that is sized to correspond to an inner diameter of a cold chamber 26. A pour port 28 facilitates the ladling of molten metal 30 into the cold chamber 26 when the plunger 22 is retracted. The metal 30 is forced by the plunger tip 24 into cavities formed in a mold or die 32.
At the beginning of a “shot” cycle, the actuating piston 18 and plunger 22 are fully retracted, allowing molten metal 30 to be ladled into the cold chamber 26 through the pour port 28. The piston 18 is then caused to advance slowly toward the die 32 (to the left in FIG. 1) so that the plunger tip 24 closes off the pour port 28. When the actuating piston 18 is in mid stroke, it continues to move at a relatively slow velocity to prevent waves in the metal 30 from trapping air. Once the plunger tip 24 has moved far enough in the direction of the mold 32 to substantially displace the air from the cold chamber 26, the plunger 22 is normally accelerated to a higher speed.
Control of the speed of the plunger 22 and plunger tip 24 typically is accomplished by controlling the speed of the actuating piston 18 through adjustment of the hydraulic inlet throttle valve 12 and the outlet throttle valve 16. The inlet throttle valve 12 normally is only partially open during the slow portion of the shot, but is opened wide at the transition to the high-speed portion of the shot. During the high-speed portion of the shot, the speed of the system typically is controlled with the hydraulic outlet throttle valve 16. Other sequences of valve control are practiced, but the resulting transition from low speed to high speed is similar.
The mold or die 32 shown in FIG. 1 includes a stationary half 34 and a movable half 36. The stationary mold half 34 and the movable mold half 36 are held together by a clamping mechanism (not shown) as known in the art. The molten metal 30 is forced from the cold chamber 26 through a main gate 38 and to a runner system 40 that leads to at least one main cavity 42. At least one overflow gate 44 extends from each main cavity 42 and each overflow gate 44 leads to an overflow chamber 46. Molds typically include multiple overflow chambers 46, with each chamber having a respective gate 44, to be described in greater detail below.
Prior to the time that the molten metal 30 reaches the main gate 38, the pressure in the runner system 40 is essentially zero. As soon as the metal 30 reaches the main gate 38, however, the restricted cross-sectional area of the main gate 38 and the high velocity of the plunger tip 24 combine to raise the pressure of the molten metal 30 in the runner system 40 to a value that might be as high as 1,000 pounds per square inch (psi). Meanwhile, until the main cavity 42 fills, the pressure therein remains relatively low. When the main cavity 42 fills and its pressure starts to rise as the hydraulic cylinder 14 continues to urge the plunger tip 24 forward, molten metal flows through the main cavity 42 and through the overflow gates 44 to the overflow chambers 46. As is well known in the die casting art, the flow of the molten metal 30 into the overflow chambers 46 assists in filling voids in the metal that is in the main cavity 42. The details of this flow will be described below.
Rapid flow of the molten metal 30 into the main cavity 38 leads to advantageous properties in the resulting casting. However, the velocity during the high-speed portion of the shot is limited by the ability of the clamping force to hold the stationary mold half 34 and the movable mold half 36 together due to the impact pressure that occurs when the main cavity 38 fills with fluid molten metal 30. Specifically, if the speed of the plunger 22 (and therefore the speed of the molten metal 30) is too high, the impact pressure of the molten metal 30 on the mold 32 will be too high. This will cause the mold halves 34 and 36 and the clamping mechanism to deflect, allowing the mold 32 to open slightly, which in turn allows some of the molten metal 30 to emerge from the mold 32, or at least to create flash. Flash is a thin film of metal that undesirably spreads out on the parting line of the mold halves 34 and 36 from the main cavity 38 and/or the runner system 40.
Because the impact pressure of the molten metal 30 on the mold 32 is proportional to the speed of the plunger 22, systems of the prior art have concentrated on control of the plunger 22 to decelerate it rapidly and thereby decrease the impact pressure of the metal 30 on the mold 32 just before the mold 32 fills. Typically, valves have been employed on the hydraulic cylinder 14 to release the hydraulic pressure on the actuating piston 18, thereby decreasing the speed of the plunger 22.
FIGS. 2 and 3 show a typical raw casting 48 as it appears when it is removed from the mold 32 (referring back to FIG. 1). The raw casting 48 essentially is a solid body defined by the cavities, chambers, gates, runners and other openings formed in the mold 32. The raw casting 48 is a symmetrical part, including a runner formation 50 (corresponding to the runner system 40 of the mold 32) leading to a main gate formation 52 on each half of the raw casting 48 and then to the actual cast part 53 that is formed in the main cavity 42 of the mold 32. The presence of multiple overflow metal formations 54, each connected to the actual cast part 53 by an overflow gate formation 56, illustrates the placement of the overflow chambers 46 and overflow gates 44 in the mold 32. In some applications, metal may flow through the overflow chamber 46 and into an air vent, creating a corresponding metal formation 58.
Referring now to FIGS. 1–3 in combination, it is well known to place overflow chambers 46 around the periphery of the main cavity 42 of a die cast mold 32. Overflow chambers 46 are required because the die casting process invariably causes a mixing of air in the initial molten metal 30 that flows through the main cavity 42, as the initial portion of the molten metal 30 pushes air ahead of it. The overflow chambers 46 provide a place for that aerated metal to flow, allowing the denser, non-aerated molten metal 30 to fill the main cavity 42 and produce a higher-quality part.
The overflow chambers 46 are located on the periphery of the main cavity 42, which is the area that the metal 30 reaches last. In addition, the restricting overflow gate 44 that leads to each overflow chamber 46 requires increased pressure in the main cavity 42 to cause the molten metal 30 to flow through each gate 44. These factors combine to keep the overflow chambers 46 relatively empty as the main cavity 42 fills.
Once the main cavity 42 fills significantly, the pressure exerted by the plunger 22 causes the metal 30 to pass through the overflow gates 44 and into the overflow chambers 46. The generation of the pressure that causes the metal 30 to flow into the overflow chambers 46 corresponds to the generation of the impact pressure described above that may cause the mold 32 to deflect.
The placement and total volume of the overflow chambers in the prior art typically has been determined by the past experience of the mold designer as well as by trial-and-error, with the objective of a placement and volume that allows the maximum amount of aerated molten metal to flow into the overflow chambers, ideally resulting in a minimal level of porosity in the final part. Another design consideration, the total cross-sectional area of the gates that lead to the overflow chambers, has generally been set equal to or approximately equal to the cross-sectional area of the main gate between the runner system and the main cavity. As a result, there has been no consideration in the prior art of designing the volume of the overflow chambers and the cross-sectional area of the overflow gates to relieve the impact pressure of the molten metal on the mold.
Accordingly, it is desirable to develop a method and an apparatus to allow the overflow chambers and overflow gates to be designed to reduce the impact pressure of the molten metal on the mold, thereby reducing the force required to clamp the mold halves and shut. The invention includes a process that reduces the metal pressure during the deceleration of the plunger that drives metal into the mold of a die-casting machine by sizing the overflow chambers and overflow gates in a new way.